It has long been recognized that reinforced ceramic engine components afford significant gains over like components fabricated from superalloys in such physical properties as high temperature strength and stiffness, and improved fatigue, creep, corrosion, and wear resistance. Unfortunately, as a practical matter, it is precisely these enhanced physical properties--particularly the significantly lower coefficient of thermal expansion (CTE) and higher operating temperature capability of ceramics over metals--that present the greatest impediment to the use of ceramics in an application such as a turbine engine nozzle.
Specifically, the primary difficulty encountered when incorporating reinforced ceramic nozzle components within a turbine engine lies in maintaining critical tolerances between the low-CTE, high operating temperature ceramic components and the relatively higher-CTE, lower operating temperature metallic structures of the engine. In an effort to overcome this problem, the prior art teaches the use of individual ceramic composite vanes installed between, and retained by, inner and outer rings formed either of ceramic or metallic material. However, close tolerance control of vane incidence angles and flowpath contours is particularly difficult, and the inner ring, or hub, requires additional supporting structures, as it cannot be supported by the vanes alone. The metallic inner supporting structures characteristically employed by the prior art must be cooled to prevent overheating, thereby limiting the advantages of the advanced material system while vastly increasing system complexity.
The disparity between the CTE of ceramics in relation to metals also presents difficulties in attaching a ceramic nozzle to the outer metallic structure of the engine. The prior art teaches the use of an annular, frusto-conical metallic support to attach a metal nozzle to the metallic structure of the engine. However, unacceptably large hoop stresses are developed in such a metallic support when utilized to support a ceramic nozzle. Indeed, the higher operating temperatures achievable with ceramics exacerbates the dimensional disparity between support and nozzle, thereby making the interface between the ceramics and metals even more problematical. Once again, the full potential of ceramics is not realized by the prior art.